1. Field of the Invention
The present invention relates in general to a leadframe, and in specific to a leadframe suitable for micro leadframe packages, and more specifically, to a leadframe having a mold inflow groove for increasing the number of potential leads on a package.
2. History of the Related Art
In the semiconductor industry, packaging techniques of integrated circuits have generally been and continue to be developed to meet the requirements of slimness and packaging reliability. The requirement for slimness forces packages to be reduced even to semiconductor chip size. In one aspect of packaging reliability, much effort has been made to develop fabricating techniques for semiconductor packages, which are improved in mechanical and electrical reliability.
A conventional leadframe comprises a frame body, tie bars internally extended from the comers of the frame body, a die pad located in the center of the frame body having a contact with the tie bars, and a plurality of inner leads extended from the frame body with the tie bar ends being located along the perimeter of the die pad. The die pad provides a site on which a semiconductor chip may be mounted.
A typical method for fabricating a semiconductor package using the above leadframe comprises several steps. First, a semiconductor chip is bonded on the die pad. Next, bonding pads of the semiconductor chip we wire-bonded to corresponding inner leads by the use of a conductive connecting member such as gold wire, so that the semiconductor chip is electrically connected to the inner leads.
In a conventional mold die equipped with a package gate-type mold gate, a cavity is formed by combining an upper cavity bar with a lower cavity bar. A mold gate is provided which is directly connected to the cavity and allows a molding compound to be charged safely and homogeneously into the cavity and a runner which is directly connected to the mold gate. Between the mold gate and the upper surface of the tie bar, there is a predetermined gap. The runner functions as a passage through which the molding compound flows.
During the molding, a leadframe is positioned within the cavity while a molding compound is charged to a port inside a cull block. As the molding compound is pressurized, the molding compound which is melted by the heat from the mold die flows through the runner, a passage formed in the upper and lower cavity, the mold gate, an inlet to the cavity, and finally into the cavity. After a period of time, the molding compound is cured to finalize the molding process.
After the molding, singulation is conducted to separate packages from the mold frame into independent individual package units. During singulation, packages are uprightly secured to the singulating apparatus and turned upside down before final singulation.
Since the mold gate is spaced at a distance from the upper surface of the tie bar, the molding compound flows through the mold gate and into the cavity. However, after completion of the molding, a thickness of a cull is left on the area where the mold gate of the leadframe is positioned.
Due to limitations of the physical equipment, micro leadframes are impossible to subject to live bug singulation. Thus, dead bug singulation must be performed.
When micro leadframes are subjected to live bug singulation, burrs are formed at the ends of leads by the action of a punch directing downwardly. In this case, mounting defects occur due to the orientation of the burrs. Therefore, after the micro leadframe is positioned in a singulation die, sides of a mold body as well as leadframe corner areas to which the tie bars extend are also subjected to dead bug singulation, thereby preventing the mounting defects attributed to the burrs.
Where a molding process is conducted using the conventional leadframe and the mold die comprising the mold gate, the molding compound which remains on the leadframe comer areas connected to the tie bars causes the tie bars to be higher in level at the mold gate than other sites, resulting in non-planarity of the mold frame/singulation die.
In the mold die applied by the conventional leadframe for micro leadframes and the package gate-type mold gate, the mold gate is directly connected to the cavity, so that the molding compound flowing through the runner is immediately introduced into the cavity. Accordingly, after the molding, a dreg of a cured molding compound remains on the upper surface of the tie bar. The mold frame thus is not horizontally positioned on the singulation die due to the thickness of the molding compound left on the tie bar, so that the dead bug singulation cannot be conducted accurately.
Attempts, such as creating tie bars having Y shapes, have been made to eliminate the above problems. Although being advantageous in the accurate singulation, these attempts suffer from several problems.
Because the tie bars of such attempts are Y-shaped, the area necessary to form the inner leads in the leadframe is reduced as much as the area occupied by the branches of the Y-shape. As a result, the number of the inner leads is decreased. The decrease in inner leads is a significant drawback in present-day applications, especially with the decreasing size of the semiconductor packages and the increasing efficiency of the same.
In an effort to overcome this and other problems encountered in the prior art, and to provide a leadframe which secures a sufficient number of inner leads for connecting with outer power sources as well as allowing singulation to be implemented accurately, a molding compound is introduced through mold inflow grooves positioned at the left and right sides of a tie bar into the cavity so that no culls remain on the tie bar after the molding.
In one embodiment of the present invention, there is provided a leadframe comprising a frame body, tie bars internally extended from the comers of the frame body, a die pad located in the center of the frame body for providing a site on which a semiconductor chip is mounted and having a contact with the tie bars, and a plurality of inner leads extended from the frame body with their ends being located along the perimeter of the die pad. Located at each comer area of the frame body is a mold inflow groove. The mold inflow groove is connected with the empty space around the inner leads. When loading the leadframe on a mold die, the end of a runner of the mold die is positioned at a distance away from a molding area of the leadframe, and the mold inflow groove is divided into two domains by each tie bar. The mold compound is introduced through the mold inflow grooves positioned on either side of the tie bar and into the cavity so that no culls remain on the tie bar after the molding.
In practice, the mold compound departs the runner; flows through the mold inflow groove at both sides of the tie bar; enters the leadframe gate formed by the mold inflow groove; travels through the upper and lower cavity bars; and fills the cavity formed by the upper and lower cavity bars.